diff --git a/Classes/Domain/Import/Importer/FetchData/InvalidResponseException.php b/Classes/Domain/Import/Importer/FetchData/InvalidResponseException.php
index cae27d9..4568a56 100644
--- a/Classes/Domain/Import/Importer/FetchData/InvalidResponseException.php
+++ b/Classes/Domain/Import/Importer/FetchData/InvalidResponseException.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 /*
  * Copyright (C) 2021 Daniel Siepmann <coding@daniel-siepmann.de>
  *
diff --git a/Classes/Domain/Import/Typo3Converter/LanguageHandling.php b/Classes/Domain/Import/Typo3Converter/LanguageHandling.php
index f7950e0..940f957 100644
--- a/Classes/Domain/Import/Typo3Converter/LanguageHandling.php
+++ b/Classes/Domain/Import/Typo3Converter/LanguageHandling.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 /*
  * Copyright (C) 2021 Daniel Siepmann <coding@daniel-siepmann.de>
  *
diff --git a/Tests/Functional/ImportTest.php b/Tests/Functional/ImportTest.php
index 6278e72..a2ea7da 100644
--- a/Tests/Functional/ImportTest.php
+++ b/Tests/Functional/ImportTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace WerkraumMedia\ThueCat\Tests\Functional;
 
 /*
diff --git a/Tests/Unit/Domain/Import/UrlProvider/SyncScopeUrlProviderTest.php b/Tests/Unit/Domain/Import/UrlProvider/SyncScopeUrlProviderTest.php
index 9c0b7f0..c6b8e82 100644
--- a/Tests/Unit/Domain/Import/UrlProvider/SyncScopeUrlProviderTest.php
+++ b/Tests/Unit/Domain/Import/UrlProvider/SyncScopeUrlProviderTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace WerkraumMedia\ThueCat\Tests\Unit\Domain\Import\UrlProvider;
 
 /*
diff --git a/Tests/Unit/Domain/Model/Frontend/AddressTest.php b/Tests/Unit/Domain/Model/Frontend/AddressTest.php
index 221b366..0530066 100644
--- a/Tests/Unit/Domain/Model/Frontend/AddressTest.php
+++ b/Tests/Unit/Domain/Model/Frontend/AddressTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace WerkraumMedia\ThueCat\Tests\Unit\Domain\Model\Frontend;
 
 /*
diff --git a/Tests/Unit/Domain/Model/Frontend/MediaTest.php b/Tests/Unit/Domain/Model/Frontend/MediaTest.php
index 328c78b..2c11f70 100644
--- a/Tests/Unit/Domain/Model/Frontend/MediaTest.php
+++ b/Tests/Unit/Domain/Model/Frontend/MediaTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace WerkraumMedia\ThueCat\Tests\Unit\Domain\Model\Frontend;
 
 /*
diff --git a/ecs.php b/ecs.php
index 1dcf8c8..f339111 100644
--- a/ecs.php
+++ b/ecs.php
@@ -39,6 +39,8 @@ return static function (ContainerConfigurator $containerConfigurator): void {
         ],
     ]);
 
+    $services->set(DeclareStrictTypesFixer::class);
+
     $services->set(NoUnusedImportsFixer::class);
     $services->set(FullyQualifiedStrictTypesFixer::class);
     $services->set(NoMultilineWhitespaceAroundDoubleArrowFixer::class);